Quote Originally Posted by the great waldo View Post
Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew
If I were you I'd buy the one from Conrad, or something similar ready made. In my opinion it is not worth the effort and is safer and guaranteed to work. The reason I linked to that circuit is because you asked for an example. Anyway, if you decide to build it then in your case I think it needs some modification, so I'd modify the following:

1. The four resistors, as Doddy mentioned already. It is better to use more powerful type, even if the 5W may work for such application because of the short time current if flowing trough. In fact, I think a single resistor would be better, something like this:

https://www.conrad.at/de/ate-electro...st-420251.html

That one is 47 Ohm, so the current will be limited a little more, to 4.9A Instead of 5.75A with the 40 Ohm, but that doesn't matter.

2. The relay in the example is 8A, I'd take a more powerful one, for example this one:

https://www.conrad.at/de/printrelais...st-502934.html

3. The fuse should be 10A slow type.

4. The art work in my opinion should have more powerful tracks, especially on the K1, K1 and the relay part. No need to waste copper by removing more than what's necessary to provide isolation.

5. Resistors R1, R2 and R3 should be metal film, 0.6W or 1W type. Even if the current trough is low, it is good practice to use better resistors than coal film type.

https://www.conrad.at/de/metallschic...t-1417688.html

https://www.conrad.at/de/metallschic...t-1417626.html